1. Field
This disclosure generally relates to lighting devices that employ active light sources.
2. Description of the Related Art
Lighting devices exist in a broad range of designs suitable for various uses. Some lighting devices illuminate interior spaces, while others illuminate exterior spaces. Some lighting devices are used to provide information, for example, forming part of or all of a display panel. Active lighting sources take a variety of forms, for example incandescent lamps, high-intensity discharge (HID) lamps (e.g., mercury vapor lamps, high-pressure sodium lamps, metal halide lamps), and solid-state light sources for instance light emitting diodes (LEDs).
Lighting devices have a number of defining characteristics, including intensity (e.g., lumens), focus or dispersion, and temperature of the emitted light. Achieving desired lighting typically requires selecting suitable light sources, lenses, reflectors and/or housings based at least in part on the defining characteristics, the environment in which the lighting device will be used, and the desired level of performance.
LEDs are becoming increasingly popular due to their high energy efficiency, robustness, and long life performance. Typically, practical LEDs are capable of emitting light in a relatively narrow band. Since white light is often desirable, solid-state lighting systems typically employ “white” LEDs. These “white” LEDs may be manufactured by placing a phosphor layer either directly on a blue emitting LED die or onto a lens or window through which an LED will emit light. The phosphor layer is typically designed to convert radiation in the 440 to 480 nanometer wavelength range into a wider spectrum consisting of longer visible wavelengths that when added to residual blue light will appear as a pleasing white light. A variety of white LEDs are commercially available from a variety of manufacturers. Commercially available white LEDs range from “cool” white with a correlated color temperature (CCT) of approximately 6000 Kelvin to “warm” white with a CCT of approximately 3000K.
Varying the composition of the phosphor controls the CCT. The phosphors used in the manufacture of the lower CCT LEDs are often less stable over the life of the LED. Additionally, the typical method of measuring the luminous efficiency of LEDs is based on the Human Eye Response, as shown by the well-known International Commission on Illumination (CIE) curve. Because the higher CCT LEDs have more light energy of the wavelengths near the center of the CIE curve, the efficacy, expressed in photometric lumens per watt, is higher than the lower CCT LEDs. For these reasons, higher CCT LEDs have been preferred for solid-state luminaire and lamp manufacture, in spite of their introduction of potential harmful effects on the environment.
The industry preference toward higher CCT LEDs has been driven by efficiency, discussed above, and lower cost, however, higher CCT LEDs emit light that has negative biological effects on the environment. Hence, it is desirable to find a relatively low cost approach to lowering CCT and “warming” the light emitted by higher CCT LEDs.
In contrast to LEDs, HIDs commonly function by sending a current through a gas (e.g., mercury, sodium). The gas may contain additive metal atoms as well as the fundamental component of the gas. Electrons, gas atoms and metal atoms are accelerated within the HID lamp bulb, colliding and releasing higher CCT light. This light includes blue light, and ultra-violet (UV) light which may result in harmful environmental effects.